1. Technical Field
The field of the invention is that of integrated circuit fabrication, in particular forming thin film resistors integrated into the back end process and having a resistance value that is stable under temperature changes.
2. Background of the Invention
Thin film resistors are utilized in electronic circuits in many important technological applications. The resistors may be part of an individual device, or may be part of a complex hybrid circuit or integrated circuit. Some specific examples of thin film resistors in integrated circuits are the resistive ladder network in an analog-to-digital converter, and current limiting and load resistors in emitter follower amplifiers.
Film resistors can comprise a variety of materials including tantalum nitride (TaN), silicon chromium (SiCr), and nickel chromium (NiCr). These resistor materials are generally evaporated or sputtered onto a substrate wafer at a metal interconnect level and subsequently patterned and etched. The thin film resistors require an electrical connection to be made to them and generally the performance of the resistors is related to the condition and cleanliness of the resistor surface and the integrity of the electrical connection. It is well known that contaminants incorporated in the resistor material and around the electrical interconnects can have adverse effects on the resistor performance. It is important to ensure that during the manufacturing process, the resistor surface is not exposed to materials and chemicals likely to leave behind contaminants on the resistor surface that will adversely affect either the bulk sheet resistivity or the subsequent interconnect areas.
A well known method of ensuring that the resistor does not come into contact with potential contaminants during processing is to deposit a sacrificial barrier layer, such as titanium(TiW) or other suitable material over the resistor just after it has been deposited. This barrier layer is often referred to as a “hard mask”. After the barrier layer and resistor material are patterned and etched, the metal for the metal interconnect is deposited, patterned and etched. The “hard mask” protects the resistor during this processing and is eventually removed by a wet chemical process such as exposure to a hydrogen peroxide (H2O2) solution just before an insulation layer or passivation layer is deposited over the resistor to permanently protect it.
A persistent problem in the art is that the temperature range over which a circuit operates can vary by a large amount and that various electrical parameters are sensitive to temperature changes.
A common technique in the art has been to construct circuits that depend on the ratio of resistors, rather than the absolute value of resistance. The benefit of this has been that it is much easier to control the ratio of areas by lithography, so that the resulting ratio of resistances is insensitive to parameters such as film thickness and film resistivity. This technique requires considerably more area than a single resistor.
In current technology, however, designers are using circuit modules that depend on the value of a resistor more directly.
It is known, for example that TaN deposited on oxide is typically a mixture of hexagonal and cubic phases and has a TCR of −650 ppm/C, which produces a wide variation in operating resistance.
TaN in the cubic phase has a much lower TCR of 300 ppm/C, but it has not been easy (practical) to control the phase of the final film after various further processing steps.
U.S. Pat. No. 6,331,811 shows a thin film resistor made from a matrix of amorphous TiN containing crystals of TiN and Ti.
U.S. Pat. No. 6,645,821 shows an integration scheme for a thin film resistor in which vias are formed simultaneously from an upper level to the resistor and to the substrate on which the resistor rests.
U.S. Pat. No. 5,485,138 shows a structure of a thin film resistor in which the film is deposited above the contacts, thereby removing the problem of etching through an upper protective layer on the top of the resistive film.
The art could benefit from a simple method of forming a thin film resistor having reduced variation in the resistance of the final product.